The present invention relates to an exchangeable solid cathode in a fused salt reduction cell for producing aluminum, in which at least one of the working faces is of a wettable material.
In the fused salt reduction process for producing aluminum from aluminum oxide the latter is dissolved in a fluoride melt comprised for the greater part of cryolite. The cathodically precipitated aluminum collects on the carbon floor of the cell under the fluoride melt, the surface of the molten aluminum forming the actual cathode. Dipping into the electrolyte from above are anodes which in conventional processes are made of amorphous carbon. At the carbon anodes oxygen is formed as a result of the electrolytic decomposition of the aluminum oxide; this oxygen combines with the carbon of the anodes to form C0.sub.2 and CO.
The electrolytic process takes place in a temperature range extending from approximately 940.degree. C. to 970.degree. C. In the course of the process the electrolyte becomes deplete in aluminum oxide. At a lower concentration of about 1-2 wt. % aluminum oxide in the electrolyte the anode effect occurs whereby there is an increase in voltage from, for example, 4-5 V to 30 V and higher. Then at the latest the aluminum oxide concentration must be raised by addition of fresh aluminum oxide (alumina). In modern reduction cells the addition of alumina takes place at short intervals via at least one opening which is kept open at all times by means of a chisel.
It is known to employ wettable solid cathodes in the aluminum fused salt reduction process. To that end cathodes made of titanium boride, titanium carbide, pyrolytic graphite, boron carbide and other materials are proposed; likewise mixtures of these, which for example can be sintered, are also employed.
In the case of wettable cathodes the interpolar gap can be reduced from the normal approximately 5 cm by as much as the normal parameters allow e.g. taking into consideration the circulation of the electrolyte in the interpolar gap, the escape of the anode gases without reoxidation by the aluminum and maintaining the pot temperature. Reducing in size of the interpolar gap results in a significant reduction in the energy consumed.
The U.S. Pat. No. 4,243,502 reveals solid cathode of individually exchangeable elements each of which has at least one lead for conducting away the current. In a further development revealed in U.S. Pat. No. 4,376,690 the exchangeable elements are of two parts which are joined mechanically and rigidly together, the said parts being resistant to thermal shock, one part viz., the upper part dipping into the precipitated aluminum from the molten electrolyte, and the other part situated completely in the molten aluminum. The said parts are made of different materials, the upper part at least in the region of the surface is unchanged in that it is made of a material which is wet by aluminum, the lower part or a coating thereon being made of an insulating material which is resistant to attack by the molten aluminum.
The German patent publication DE-OS No. 31 42 686 reveals a solid cathode which can be employed in a cell for producing aluminum by the fused salt reduction process, said cathode featuring in its make up an aluminide of at least one transition metal of the groups IV A, V A, and VI A of the periodic table of elements. This solid cathode comprises essentially a supporting part and a structure which at least in the region of the working surface features open pores and is impregnated with aluminum saturated with a transition metal or transition metals. This open pore structure can be fed continuously from a reservoir of aluminide or aluminides. It was found that a few millimeters thick felt pad of carbon fibers proved to be a particularly advantageous type of open pore structure. According to a special version of the said cathode the density of the cathode can lie between that of the molten aluminum and that of the electrolyte thus causing the solid cathode to float in the electrolyte.